Method and a device for bending compensation in intensity-based fibre-optical measuring systems

ABSTRACT

Compensation is provided for bending of an optical fibre in intensity-based optical measuring systems. A measuring signal and a reference signal of different wavelengths are generated and transmitted through an optical connection towards a sensor element. The reference signal is not influenced in the sensor element. The measuring and reference signals are detected and compensation is carried out for bending of the optical connection using correction data. The correction data is based upon pre-store data concerning the relationship between the measured reference signal and the measured measuring signal as a function of the bending influence on the optical connection. Devices and methods according to the invention allow for measurements with an optical pressure measuring system that exhibit effective compensation for any bending of the optical connection.

TECHNICAL FIELD

The present invention relates to a method for measuring systems intendedfor use with intensity-based fibre-optical measuring systems forpressure measurements. The invention also relates to a device forcarrying out such a method.

BACKGROUND ART

In connection with measuring physical parameters such as pressure andtemperature, it is previously known to utilise various sensor systems bywhich the optical intensity of a ray of light, conveyed through anoptical fibre and coming in towards a sensor element, is influenced dueto changes in the respective physical parameter. Such a system may forexample be used when measuring the blood pressure in the veins of thehuman body. Said system is based upon a transformation from pressure toa mechanical movement, which in turn is transformed into an opticalintensity, conveyed by an optical fibre, which is in turn transformedinto an electrical signal that is related to the measured pressure.

According to known art, such a fibre-optical measurement system maycomprise a pressure sensor, an optical fibre connected to said pressuresensor, and at least one light source and at least one light detectorlocated at the opposite end of the fibre, in order to provide thepressure sensor with light, and to detect the information-carrying lightsignal returning from the pressure sensor, respectively.

One problem occurring with previously known systems of the above kindrelates to the fact that interference may occur in the signaltransmission path, for example caused by fibre couplings or throughbending, intentionally or unintentionally, of the fibre. Already at alight deflection of the fibre, a reduction of the light signal occurs.This signal damping, caused by the bent fibre, entails that the lightsignal detected in the light detector, which is related to the pressuredetected in the sensor element, will have a value that does not coincidewith the real pressure. The size of the deviation will then depend onhow much the fibre was deflected.

Through EP 0 528 657 A2 a fibre-optical measurement system for measuringpressure is known. Said system comprises a pressure sensor with amembrane, three LED:s emitting light at different wavelengths, and twophoto detectors. The system is arranged so that a computing algorithm isused for correction of such temperature effects that may have beensuperimposed on the output pressure signal. This algorithm is based uponthe relationship between membrane deflection, pressure and temperature.Correction data obtained experimentally may also be used as input datato the algorithm regarding temperature compensation.

DISCLOSURE OF INVENTION

A primary object of the present invention is to compensate, by means ofa method and a device, for interference in intensity-based fibre-opticalsensor systems, caused by intentional or unintentional bending of theoptical fibre. This is achieved by means of a method and a device inaccordance with the present invention.

The invention is intended for bending compensation in intensity-basedoptical measurement systems comprising a sensor element connected to ameasuring and control unit via an optical connection and adapted forproviding a signal corresponding to a measurement of a physicalparameter in connection with the sensor element. The inventioncomprises; the generation of a measuring signal that is brought to comein towards the sensor element; the generation of a reference signal thatis transmitted through the optical connection without being influencedin the sensor element, said measuring signal and said reference signalhaving different wavelengths; and the detection of said measuring signaland the detection of said reference signal. The invention ischaracterised by comprising bending compensation through correction databased upon pre-stored data concerning the relationship between themeasured reference signal and the measured measuring signal as afunction of the bending influence on said optical connection.

Advantageous embodiments of the invention are defined by the subsequentdependent claims.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be explained in more detail below, with reference toa preferred embodiment and to the enclosed drawings, in which:

FIG. 1 shows, schematically, a pressure measuring system according tothe present invention;

FIG. 1 a shows an enlarged view of a sensor element intended for use inconnection with the invention;

FIG. 2 shows a graph illustrating the relationship between a measuredreference signal and a measured measuring signal as a function of thebending influence, in accordance with a method according to theinvention; and

FIG. 3 shows, in principle, a pressure measuring system in which aso-called “smart card” can be used as the information-carrying memoryunit.

PREFERRED EMBODIMENTS

FIG. 1 shows, schematically, an intensity-based fibre-optical measuringsystem 1 according to the present invention. According to a preferredembodiment, the arrangement is used in connection with a fibre-opticalmeasuring system of an as such previously known kind, which couldpreferably, but not exclusively, consist of a pressure measuring system.Alternatively, the invention could be used e.g. for measuringtemperature and acceleration.

Two light sources belong to the system 1, comprising a first LED 2 and asecond LED 3, the first LED 2 functioning to emit a first light signalof a first wavelength λ₂, and the second LED 3 functioning to emit asecond light signal of a second wavelength λ₂, said wavelengths beingdifferent. The LED:s 2, 3 are connected to an optical conduit,preferably in the form of an as such previously known optical fibre 4,by means of a first link 5 and a second link 6, respectively, and alsovia a fibre coupling 7. The optical fibre 4 is connected to a sensorelement 8, schematically illustrated in FIG. 1.

According to what is shown in detail by FIG. 1 a, which is an enlargedview of the sensor element 8, said element comprises a cavity 8 a, forexample obtainable (according to known art) through construction bymeans of molecular layers (primarily silicone, alternatively siliconedioxide or a combination of the two) and an etching procedure.Preferably, a bonding procedure is also utilised in assembling thevarious layers of the sensor element 8. The manufacture of such a sensorelement 8 is as such previously known, e.g. from the patent DocumentPCT/SE93100393. In this way, a membrane 8 b is also created within thesensor element 8, the deflection of which membrane will depend on thepressure p surrounding the sensor element 8.

According to what will be described in detail below, the first lightsignal with the first wavelength λ₁ will come in and be reflectedagainst the cavity 8 a within the pressure sensor 8, whereas the secondlight signal with the second wavelength λ₂ is brought to come in ontothe bottom side of the sensor element 8, i.e. towards the interfacebetween the pressure sensor 8 and the optical fibre 4. Hereby, the firstlight signal will be modulated by the pressure p acting on the membrane8 b. When the membrane 8 b is influenced, the dimensions of the cavity 8a, primarily its depth d, will change, entailing a modulation of thefirst light signal through optical interference inside the cavity 8 a.

The second light signal will be reflected against the bottom side of thesensor element 8, due to the fact that the silicone defining the sensorelement 8 will only allow transmission of light with a wavelength longerthan a certain limit value (e.g. 900 nm). Consequently, said firstwavelength λ₁ will be selected so as to exceed this limit value.Contrary to this, said second wavelength λ will be selected so as tofall below this limit value. After having determined the two wavelengthsλ₁, λ₂, appropriate dimensions of the cavity 8 a are determined. Forexample, the depth of the cavity 8 a is selected to be a value ofsubstantially the same magnitude as the two wavelengths λ₁, λ₂. Thesizing of the cavity 8 a is made considering the required applicationrange for the sensor element 8 (in the current case primarily thepressure range to which the sensor element 8 is to be adapted).

The light signal (λ₁) emitted from the first LED 2 defines a measuringsignal that is thus transmitted through the fibre 4 to the sensorelement 8, where said light signal will be modulated in the mannerdescribed above. The second light signal (λ₂) will then define areference signal, transmitted through the fibre 4 and being reflected bythe bottom side 9 of the sensor element 8. The light signal modulated inthe sensor element 8 and the light signal reflected from the bottom side9 of the sensor element are then transmitted back through the fibre 4.The returning light signals will, through the fibre coupling 7, beconveyed into fibre links 10, 11, connected to the detectors 12 and 13,respectively. The detectors 12, 13 will detect the measuring signal andthe reference signal, respectively.

The four links 5, 6, 10, 11 preferably consist of optical fibres, thefibre coupling 7 thereby preferably consisting of an as such known fibrejunction device designed so as to transfer the four fibre links 5, 6,10, 11 into the fibre 4 leading to the sensor element 8.

The system 1 also comprises a computerised measuring and control unit14, to which the LED:s 2, 3 and the detectors 12, 13 are connected. Saidunit 14 comprises means for processing the values detected by saiddetectors 12, 13. According to the invention, the processing of thedetected values includes a compensation for intentional or unintentionalbending of the fibre 4, by utilising correction data based uponpre-stored data concerning the relationship between a measured referencesignal and a measured measuring signal as a function of the bendinginfluence on the optical fibre 4. Such correction data could for examplebe comprised of a table or a function defining values to be used duringmeasurements to correct the detected measuring signal.

Finally, the system 1 comprises a presentation unit 15, e.g. a display,allowing a measurement of the sensed pressure p to be visualised for auser.

FIG. 2 graphically illustrates how the above relationship between ameasured reference signal and a measured measuring signal is changedduring increased bending of the fibre 4. In the figure, the referencesignal is referenced as “Output signal λ₂ [V]” and the measuring signalas “Output signal λ₁ [V]”. Said measured relationship can be describedby a function, so as to correct the measuring signal continuously with aspecific value depending on the reference signal. Alternatively, themeasured relationship can be used for defining a mathematical function,which in turn is used for producing corrected values during measurementswith the system according to the invention. As a further alternative, anumber of measurement values may be registered in a table, into whichthe value of the reference signal is entered, to obtain a value (withthe aid of interpolation, if necessary), with which the currentmeasuring signal is corrected. Independently of the correction procedureused, it is performed in the above-mentioned measuring and control unit14.

FIG. 3 shows, in principle, a pressure measuring system according to theinvention, comprising an alternative measuring unit 16 to which thesensor element 8 is connected, via the optical fibre 4, in anexchangeable manner via an optical coupling (not shown in FIG. 3). Saidmeasuring unit 16 also comprises a reader unit 17 for insertion andreading of a separate unit in the form of an information-carrying card18 (also called “smart card”). Said card 18 comprises a memory devicewhere data regarding the sensor element 8 are stored for use. Duringmeasurements, these data may be read by the measuring unit 16 and beused for example for bending compensation in dependence of whichspecific sensor element 8 that is being used for the moment. Theinvention thus provides a further advantage, in that different sensorelements 8 can be connected to said unit 16 without calibration, thanksto data stored on the information-carrying card 17. Said data preferablydefine the relationship between predetermined correction data, producedthrough measurements of the first as well as the second light signal atvarious degrees of bending of the optical fibre.

The invention is especially suitable in case a single measurementstation with one measuring unit 16 is used together with severalexchangeable sensor elements. In such a case, data corresponding toproperties, measuring range, etc. of each sensor element, can be storedon a corresponding number of information-carrying cards, each thencorresponding to (and being used together with) a specific sensorelement.

As an alternative to an information-carrying unit in the form of a card,the invention can also be used with other types of separate datacarriers. Further, the measuring system according to FIG. 3, as opposedto what is shown in FIGS. 1 and 2, is not limited to measurements of thekind using two different wavelengths, but can also be used whenmeasuring with for example only one wavelength.

It should be mentioned, that the card 18 may also contain other storedinformation than that mentioned above, e.g. information regarding thesensor type, calibration data, etc. The basic principle is, however,that the card 18 is coordinated with a specific sensor element such thatit will comprise stored data regarding the function of the specificsensor element. Preferably, the card 18 will be provided withinformation—in the form of a set of parameters—allowing the propertiesof the sensor element 8, together with the properties of the measuringunit 16, to provide a suitable linearisation of the characteristics ofthe specific sensor element during measurements.

The invention is not limited to the embodiment described above, but maybe varied within the scope of the appended claims. For example, theprinciple for data storage regarding a specific sensor on a separateinformation-carrying card can be used also for systems not intended forpressure measurements.

1. A method of compensating for bending of an optical fibre in lightintensity-based optical measuring systems, said light intensity-basedoptical measuring systems comprising a sensor element connected to ameasuring and control unit via optical fibre and being adapted forproviding a signal corresponding to a measurement of a physicalparameter, said method comprising generating a measuring light signal;transmitting the measuring light signal through the optical fibretowards the sensor element; generating a reference light signal;transmitting the reference light signal through the same optical fibrewithout being affected by the sensor element due to the measuring lightbeing separated from the reference light, wherein said measuring lightsignal and said reference light signal have different wavelengths;detecting said measuring light signal after being influenced by thesensor element; detecting said reference light signal after beingtransmitted through the optical fibre and after being reflected by saidsensor element; compensating for bending of the optical fibre byreference to correction data based upon pre-stored data concerning arelationship between the measured reference light signal and themeasured measuring light signal as a function of the bending influenceupon said optical fibre, wherein said measuring light signal causesoptical interference in a cavity associated with the sensor element. 2.The method according to claim 1, wherein said correction data includes astored table or function, describing a relationship measured beforehand,between the reference light signal and the measuring light signal, as afunction of the bending influence.
 3. A method according to claim 1,wherein said sensor is utilized for pressure measurements, said sensorelement including a membrane being affected by the pressure surroundingthe sensor element.
 4. The method according to claim 1, furthercomprising guiding the measuring light signal into the cavity of thesensor element; and reflecting the reference light signal from thesensor element without entry into the cavity.
 5. The method according toclaim 1, wherein characteristics of material forming at least onesurface of said cavity permits guiding the measuring light signal intothe cavity and causes reflectance of the reference light signal from thecavity.
 6. The method of claim 1, wherein the wavelength of themeasuring light signal is selected to exceed a limit value and thewavelength of the reference light signal is selected to be less thansaid limit value.
 7. The method of claim 6, wherein said limit value isbased on a characteristic of the senor element material.
 8. The methodof claim 6, wherein dimensions of the cavity are determined based on aselected wavelength of the measuring light signal, a selected wavelengthof the reference light signal and the limit value.
 9. A device formeasurements in optical measuring systems comprising: a sensor elementadapted for providing a signal corresponding to a measurement of aphysical parameter in connection with the sensor element; an opticalfibre connected to the sensor element; a first light source and a secondlight source arranged at the opposite end of the optical fibre andfunctioning to emit a first light signal and a second light signal,respectively, at different wavelengths, the first light signal defininga measuring signal, transmitted towards the sensor element through theoptical fibre, and the second light signal defining a reference signal,transmitted through the optical fibre without being affected by thesensor element due to the measuring light being separated from thereference light; a first detector intended to detect a light signalmodulated by the sensor element; a second detector intended to detect alight signal reflected by the sensor element; and a measuring andcontrol unit, to which said detectors are connected, whereby saidmeasuring and control unit comprising means for processing the valuesdetected by said detectors, means for storing data concerning therelationship between the measured reference signal and the measuredmeasuring signal as a function of the bending influence upon saidoptical connection, and means for correcting the value detected by thefirst detector in dependence of said correction data, wherein saidsensor element comprising a cavity, shaped so as to create opticalinterference when feeding said measuring signal into the cavity.
 10. Thedevice according to claim 9, wherein said cavity includes a plurality ofmolecular silicone and/or silicone dioxide layers which have beenetched.
 11. The device according to claim 10, whereby said cavityincludes bonding layers.
 12. The device of claim 9, wherein themeasurement light signal is guided into the cavity, whereas thereference light signal is reflected by the sensor element withoutentering the cavity.
 13. The device of claim 9, wherein characteristicsof material forming at least one surface of said cavity permits guidingof the measuring signal into the cavity and causes reflectance of thereference signal from the cavity.
 14. The device of claim 9, wherein thewavelength of the first light signal exceeds a limit value and thewavelength of the second light signal is less than the limit value. 15.The device of claim 14, wherein said limit value is based on acharacteristic of the sensor element material.
 16. The device of claim14, wherein dimensions of the cavity are determined based on the firstwavelength, the second wavelength and the limit value.